River Dynamics and Channel Morphology

Pressure and Sediment Transport

• Overview of the field site with river water inflow and a reservoir.
• Three experiments: free-flowing conditions, river conditions, and dam removal.
• Sediment transport is the central theme.
• Relate to previous lectures on alluvial terraces and delta formation.
• Delta formed in the reservoir partly eroded after dam removal, leaving remnants.

Dam Removal

• Dam removal video to illustrate principles. Focus on channel shapes.
• Water velocity is critical for sediment transport.
• Deltas form in reservoirs, not river mouths.
• Lowering reservoir levels after dam removal causes delta erosion and sediment transport downstream.
• Yellow Wah dam removal was slow, with gradual chipping and flow diversion.
• Video shows a rapid dam breach using explosives in a rural area near the Columbia River.
• Rapid reservoir drawdown observed in real-time.

Sediment Dynamics

• Super sediment-plain water moves quickly with high suspended load.
• Delta surface erosion as the channel cuts through it.
• Terraces remain above the new channel level.
• Sediment trapped over the dam's lifetime is significant.
• The new channel incises and erodes sediment.
• Trees drowned when the reservoir was constructed become visible.
• Winter floods can mobilize loose sediment downstream.
• Revegetation stabilizes terraces over time, redistributing sediment.

Short-Term vs. Long-Term Impacts

• In the first year after dam removal, sediment discharge can be very high.
• High sediment levels act like sandpaper in the water, negatively impacting fish.
• Long-term benefits include removing barriers to fish passage and reintroducing sediment, improving habitat complexity.

Environmental Impact and Considerations

• Environmental impact statements consider services provided by the dam (hydropower, recreation, navigation).
• Evaluates costs and benefits of dam removal.
• Each system is unique; there's no universal decision.
• Sediment accumulation varies; contamination is a concern.
• Downstream impacts on humans and loss of services are considered.

Case Studies

• Elwha and Klamath River dams had negligible hydropower production.
• The owner decided benefits did not warrant continued operation.
• Framingtouli Dam probably won't be removed due to its benefits.
• Fish quickly reorient to the new habitat post-dam removal.
• Reproductive success improves over a few years.

River Modification

• Over 99% of rivers and streams in the US are altered.
• Dam removal is an unusual intervention.
• Prioritization aims to balance benefits by reducing generation capacity and considering tribal treaty rights.
• Dam removal is gaining traction but involves relatively small structures.
• Shifting societal views on the purpose of rivers affect dam removal decisions.
• Main stem Columbia River dams are unlikely to be removed soon.
• White Salmon River dam removal was a win for fish habitat at a low cost.

Trade-offs of Dams

• Dams are impressive engineering feats with societal benefits.
• They also have costs, requiring prioritization.
• Cheap electricity and food production are subsidized by dams.
• Controversy arises as dams require relicensing with new fish passage requirements.
• Economic costs of hydropower vs. fish passage influence decisions.
• Upgrades are expensive; efficacy varies.
• Philosophical questions arise regarding the ethical implications of dams on rivers.
• Scientific questions persist regarding fish health and passage.

Enforcement and Regulation

• Federal and state authorities enforce regulations on projects using federal money.
• Environmental laws in the 1960s and 1970s allowed citizens to sue the federal government to ensure protection of resources.
• Prior to 1970, citizens had limited direct checks on federal agencies.
• Dams were a major catalyst for the environmental movement.

Dam construction

• Rivers are diverted using coffer dams.
• Construction begins in the diverted channel.
• Hoover Dam construction involved untested techniques and risks.
• New Deal engineering projects aimed to create jobs, but many people died.
• Dam failures occur; Keaton Jamfel on YouTube provides examples.
• Reservoirs can depress the earth's surface.

Channel types

• Flowing water erodes sediment, with faster water eroding more and carrying larger particles.
• As water slows, particles deposit, influencing channel shapes and water movement.
• Three channel types are explored: straight, meandering, and braided, influenced by velocity changes.
• A single river can exhibit multiple channel types depending on location and constraints.
• Canoeists in meandering channels are influenced by channel features.

Straight Channels

• In straight channels, water flows in a straight line with spatial velocity variations.
• Faster water is in the center, slower on the sides.
• Friction occurs at banks and the riverbed, creating slower water zones along the wetted perimeter.
• The fastest water is just below the surface in the deepest part of the channel, away from friction sources.
• Faster water in the center erodes material, maintaining the channel.
• Deposition is less likely in the fastest-moving part of the channel.
• The thalweg is the deepest part of the channel with the fastest-moving water and least deposition.
• Straight channels are rare in nature, requiring stability or human intervention.
• Natural straight channels are caused by strong geologic controls.
• Human-straightened channels have levees to confine the channel.

Meandering Channels

• Disturbances in natural systems alter flow. An obstruction, like a fallen tree, causes water to bend.
• Water is forced to bend around the tree, it erodes the opposite bank.
• This deepens and widens the channel section. Faster water erodes more, creating a positive feedback loop.
• Water slows at the tree, depositing sediment.
• A bar forms, constricting the area where water can flow.
• The channel meanders, creating bends as water swings to the curve's outside.
• Faster water erodes; slower water deposits sediment.
• Meandering channels require disturbance and space to move.
• The thalweg is not in the center; it's on the eroding side.
• Fastest water promotes erosion; deposition occurs elsewhere.
• Straight channels become unstable without human maintenance.
• Human intervention maintains channels with boulders, concrete walls, and levees.
• Thalweg is in the center; slow water is along banks.
• {Channel dredging} deepens channels for larger ships, particularly on the Columbia River.
• Dams on the Columbia help navigation by creating calmer river sections for barges.

Erosional and Depositional Features

• Meandering systems create erosional and depositional features or mini landforms formed by erosion or deposition.
• Glaciers also produce erosional and depositional features.
• Meandering is when the channel length is 50% greater than a straight line between flow points.
• Thalweg moves back and forth, water hugs the bank, deepening the channel, which creates a cut bank or an erosional feature.
• River sediment deposits to create a point bar. On the side of slower movement, a zone likely to deposit sediment.
• Pairs of point bars and cut banks alternate downstream.
• Erosion cuts through riverbanks, abandoning old meanders, resulting in oxbow lakes and meander scars.
• Oxbow Lakes are old channel portions being filled with water. Meander scars are depressions in the landscape from old rivers not filled with water.
• Water depth is shown based on lighter and darker blues, with darker blue for deep water and shallow water for light blue.
• A nice profile that shows a fool wage on a straight section, our fool wage in the middle.
• The length
  of
  one
  shaped
  meander
  is
  six
  times
  the
  width
  of
  the
  channel.
• Oxbow lakes form when a river cuts through a meander neck, abandoning the old loop.
• Features of include: meander scars, oxbow legs,
  point bars, cut banks
• A uniform surface never occurs and human modification is often messily superimposed.
• Dams help control floods, which is a problem if wanted to go up next river.
• High water flows and overflows the banks. Thin film slows it down deposits sediments. River and streams result in rich soils from floods.
• There are natural levees and human levees to constrain the channels so as to prevent flooding.
• In order for Oxbow Lakes to permanently disappear water is required to evaporate.
• Rivers political boundaries can cause a problem for state properties and state lines when rivers change or disappear.
• Yazoo Tributaries are a symptom of flooding collecting water and natural rivers being a problem.
• Rivers move across the landscape over time, which is important to note.

Braided Channels

• This channel is an overloaded system with deposition always occurring caused by plentiful sediment or slow moving water.
• It is common in the headwaters of glacially fed rivers due to glaciers eroding material off the landscape.
• It is seen below glacial peaks and high in glacial milk water, leading to sediment.
• Temporal patterns occur caused by snow and ice melt which causes run off resulting in discharge.
• Flowing water is trying to pick the most efficient path through constant deposition. It flows, it slows, it deposits.
• The water is going to channel the next easiest way. A rate exists as water picks that channel immediately and as discharged increases or increases the channel width might increase.
• The channel type is common in glacial systems that are really minimal.
• The gradient of the channel is steepness from one point or another and where areas are flat, water moves slower.
• Desert streams are good examples of channel gradient and very variable in precipitation.
• There are little islands or bars that vegetated in sediment where vegetation causes stabilization while disturbance prevents stabilizing vegetation.
• A channel width exists is and they're common and areas with plentiful mobile sediment located near downstream glasses